1. Field of the Invention
The present invention relates to an electromagnetic interference suppressing device and related electronic device, and more particularly, to an electromagnetic interference suppressing device capable of effectively suppressing electromagnetic interference by enhancing completeness of a return path among electronic components and related electronic device.
2. Description of the Prior Art
With the rapid progression of technology, various high speed signal transmission interfaces, such as Digital Visual Interface (DVI) and High Definition Multimedia Interface (HDMI), etc. are being developed to meet the demand for higher resolution video display. However, electromagnetic interference at high frequencies and high energy occur when a large number of signals are transmitted through the high speed signal transmission interface. Consequently, a proper scheme for improving electromagnetic compatibility (EMC) and electromagnetic interference (EMI) of the transmission interfaces is desirable.
In general, electromagnetic interference may occur in many situations during signal transmission. For example, impedance mismatch between transmission lines and electronic components, or discontinuities occurring within bodies of electronic components can cause noise energy radiation. Usually, electromagnetic interference is generated in connections between circuit boards, connectors, integrated circuits, and even between different layers of a circuit board. A conventional method for suppressing electromagnetic interference uses an electromagnetic interference shielding material, such as a spring or a gasket, to reduce transfer of the noise energy. However, these conventional methods of preventing electromagnetic interference take up space and are expensive, without providing a laudable suppression effect.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a motherboard 10 in the prior art. The motherboard 10 employs a multilayer printed circuit board design. The motherboard 10 has a signal connector 104 installed on the motherboard 10 for transmitting or receiving signals to/from an external device through connection with an external connector 102. As shown in FIG. 1, the signal connector 104 includes an insulation body 106, a conducting terminal 108, and a metal housing 110. The insulation body 106 is coupled to the conducting terminal 108. The conducting terminal 108 is utilized for transmitting signals. The metal housing 110 is utilized for implementing a return path of signal through bosses A, B (not shown in FIG. 1) on the motherboard 10.
In the prior art, the conventional method for suppressing electromagnetic interference usually uses a spring or a gasket on the metal housing 110 to prevent electromagnetic interference. However, completeness of the return path between the motherboard 10 and the signal connector 104 is often overlooked. The signal connector 104 is bound to the grounding layer of the upper layer of the motherboard 10 by only the small-area bosses A, B, and the grounding layer of the upper layer of the motherboard 10 connects with the lowermost layer of the motherboard 10 via an internal path. However, the connection approach has various non-ideal factors, such as difference of impedance characteristics among different layers, different layout locations and different layout areas of the grounding layer, and different thicknesses of different layers, which result in discontinuities along the return path. Therefore, the signal transmission path may not be an optimum (shortest) path, but instead corresponds to the transmission environment, increasing voltage offset, as well as incurring extra electromagnetic interference caused by the lengthened return path. Further, the longer return path results in a transmission line effect. In other words, along the signal transmission path, existing impedance variations may cause signal reflection due to impedance mismatch, so that the incident wave and the reflected wave will exhibit a standing wave effect. When the standing wave ratio is large, both radiated and conducted electromagnetic interference may occur, reducing signal integrity. Thus, a solution is needed for enhancing completeness of the return path for the multilayer motherboard 10 to reduce electromagnetic interference.